1. Field of the Invention
The present invention relates to dental restorations fabricated of low-fusing overlay porcelain and to porcelain having dual use for all-ceramic and porcelain-fused-to-metal ("PFM") restorations and to dental restorations fabricated of all-ceramic cores comprising cubic leucite.
2. Brief Description of the Related Art
Porcelain dental restorations, such as crowns, bridges, and the like are highly favored because the porcelains provide strength, wear resistance, and favorable aesthetics. Older porcelain restorations generally comprise at least one porcelain layer on a metal framework, commonly known as PFM restorations. Typically, PFM restorations are fabricated by applying a dental porcelain powder in aqueous slurry to a metal alloy framework, then firing the porcelain at high temperature to form a tight, impervious porcelain layer having the appearance of natural dentition. Those skilled in the art recognize that it is important that the firing temperature of the porcelain be at least 100.degree. C. below the solidus temperature of the alloy used as the metal framework, to prevent melting or distortion of the metal framework. For example, selected gold alloys having gold contents above 70 percent exhibit solidus temperatures of about 900.degree. C. and require overlay porcelain having sufficiently low firing temperatures. It is further important that the coefficient of thermal expansion (CTE) of the porcelain be only slightly less than that of the metal so that no cracks are produced in the porcelain layer due to thermal expansion mismatch stress occurring during firing and cooling down. Metal alloys heretofore employed in the manufacture of dental restorations have typically possessed moderately high coefficients of thermal expansion ranging from about 13.times.10.sup.-6 /.degree. C. to about 15.times.10.sup.-6 /.degree. C., with the exception of titanium and some gold-based alloys which have coefficients of thermal expansion of about 9.times.10.sup.-6 /.degree. C. and 16-17.5.times.10.sup.-6 /.degree. C., respectively.
In commonly assigned U.S. application Ser. No. 08/532,179 filed Sep. 22, 1995, now abandoned, the contents of which are incorporated by reference herein, a dental porcelain composition is described which is amorphous, i.e., single phase, and which possesses a moderately high coefficient of thermal expansion closely matching those of conventional alloys and some ceramics heretofore employed in the manufacture of dental restorations. This composition is advantageously applied to such conventional alloys to provide an extremely smooth, fused glassy surface on the resulting dental restoration. Newer restorations, however, generally comprise a ceramic core in place of the traditional metal, with at least one additional porcelain layer. These are commonly referred to as "all-ceramic" systems, and can provide even better aesthetics than the metal-porcelain systems. Among all-ceramic systems, high strength porcelains provide a more natural translucency and therefore much improved aesthetics. Dental ceramics exhibit a wide range of coefficients of thermal expansion, from as low as about 8.times.10.sup.-6 /.degree. C. (e.g., alumina) to as high as about 18.times.10.sup.-6 /.degree. C. (e.g., some leucite-reinforced ceramics).
Among the commercially-available all-ceramic systems, many are based on pressable, high-strength feldspathic porcelains, for example pressable leucite-reinforced porcelains commercially available under the trade name "OPC.RTM." from Jeneric.RTM./Pentron.RTM., Inc. (Wallingford, Conn.). These feldspathic glass-ceramics comprise from about 40% to 50% of a discontinuous, evenly dispersed, tetragonal potassium leucite phase, which imparts strength to the dental restoration. Leucite is a crystalline potassium aluminum silicate (K.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) which ordinarily has a tetragonal crystal structure at room temperature. Use of tetragonal leucite, also known as "low leucite", is described for reinforcement of feldspathic dental porcelains in U.S. Pat. No. 3,052,982 to Weinstein et al., U.S. Pat. No. 4,604,366 to Kacicz et al., U.S. Pat. No. 4,798,536 to Katz, and U.S. Pat. No. 5,614,330 to Panzera, the entire contents of the foregoing patents being incorporated herein by reference. While well-suited for their intended purposes, prior art porcelains for all-ceramic restorations are available in a limited range of maturing temperatures and CTEs, and contain leucite having at least some coarse-grained morphology, that is, a distribution of grain sizes wherein at least a fraction of the grains are greater than about 10 microns, or even greater than about 20 microns. Such coarse-grained leucite can wear away the opposing natural dentition in the mouth.
There accordingly remains a need in the art for dual purpose porcelain systems wherein the maturing temperature is low enough to match that of metal frameworks, including gold alloys and porcelain cores, and even more advantageously, wherein the CTE may be adjusted to match a range of metal substructure or all ceramic cores. There particularly remains a need for high-strength porcelain systems having low maturing temperatures, yet moderately high CTEs, and having a fine-grained leucite crystal structure for reducing wear of the opposing natural dentition. Such porcelains must farther be simple and inexpensive to manufacture.